The ideal biological imaging system would provide non-invasive, high-resolution, high-sensitivity, three-dimensional (3D) functional and anatomic images of living systems. Positron Emission Tomography (PET) offers high sensitivity to a range of biological processes through the use of targeted radiolabeled probes. Unfortunately, precise signal localization can be extremely difficult due to low spatial resolution and the lack of anatomical information in PET. Magnetic Resonance Imaging (MRI) can provide high spatial resolution and excellent soft tissue contrast, but suffers from poor signal strength leading to low sensitivity. The overall goal of the proposed effort is to combine the best features of PET and MR modalities for small animal studies into a single imaging instrument that will record data in both imaging modalities simultaneously. The most immediate benefit of the integrated system will be to provide high-resolution anatomical context for PET studies. This will lead to better overall understanding of the origin of the PET signals. The 'in-register' MR images may be used to compute scatter and attenuation in the PET images and also to estimate partial volume errors in the lower-resolution PET scans, thus aiding quantification of the PET signal. The integrated system will open up a number of opportunities not possible with current independent PET and MRI scanners. [unreadable] [unreadable] [unreadable]